Regulations are becoming stricter every year regarding substances such as PM (particulate matter), NOx, CO, and HC exhausted from diesel engines. With such regulations becoming stricter, it has been difficult to meet the regulated values by only improving the engines. Consequently, technologies have been adopted wherein an exhaust gas post-treatment device is installed in the exhaust passage of the engine, and the amounts of these substances exhausted from the engine are reduced.
This being the case, a variety of research and proposals have been made regarding the use of NOx catalysts for elimination by reduction of NOx (nitrogen oxides) from the exhaust gas of internal combustion engines such as diesel engines and certain types of gasoline engines, as well as various combustion equipment. One of these involves using an NOx occlusion reduction catalyst as an NOx reduction catalyst for the diesel engines. By using the NOx occlusion reduction catalyst, the exhaust gas can be effectively purified of NOx therein.
The NOx occlusion reduction catalyst is formed as a monolith honeycomb or the like. On the support body of the monolith honeycomb are formed a large number of polygonal cells, the support body being formed of a structural material of cordierite, silicon carbide (SiC), or stainless steel. On the wall surfaces of these cells there is provided a porous catalyst coat layer acting as the catalyst support layer and formed of alumina (Al2O3) or the like. The surface of the catalyst coat layer supports a catalyst noble metal having an oxidation function, such as platinum (Pt), and an NOx occluding agent (NOx occlusion substance, NOx occlusion material, NOx absorbing agent) formed of several of alkali metals, alkaline earth metals, and rare earth elements having an NOx occlusion function. Such alkali metals include potassium (K), sodium (Na), lithium (Li), and cesium (Cs). Such alkaline earth metals include barium (Ba) and calcium (Ca). In addition, such rare earth metals include lanthanum (La) and yttrium (Y). With these components, two functions of NOx occlusion and NOx release/purification are exhibited, depending on the oxygen concentration in the exhaust gas.
Additionally, the NOx occlusion reduction catalyst occludes the NOx with the NOx occluding agent during regular operation. When the occlusion ability of the catalyst nears saturation, the air-fuel ratio of the inflowing exhaust gas is put into a rich state at an appropriate time, whereby the NOx occluded by the catalyst is released, while at the same time the released NOx is reduced by the three-way action of the catalyst noble metal.
More specifically, in the case where the air-fuel ratio of the exhaust gas is in a lean state where oxygen (O2) is present in the exhaust gas of ordinary diesel engines or lean combustion gasoline engines, nitrogen monoxide (NO) exhausted from the engine is oxidized to nitrogen dioxide (NO2) by the oxidation catalytic function of the catalytic noble metal using the oxygen present in the exhaust gas. Subsequently, the nitrogen dioxide is occluded in the form of a salt such as nitrate by the NOx occlusion agent having the NOx occlusion ability, such as barium, thus purifying the NOx.
However, when this state is continued, the NOx occluding agent having the NOx occlusion ability will be entirely transformed into the nitrate, and the NOx occlusion ability will be lost. Consequently, an exhaust gas with an exceedingly high concentration of a fuel (rich spike gas) is created by changing the operational parameters of the engine or by injecting the fuel into the exhaust passage, and this rich spike gas is then sent to the catalyst. The rich spike gas is a high-temperature exhaust gas having a high concentration of reducing agents such as carbon monoxide (CO) and hydrocarbons (HC), wherein oxygen is not present.
When the exhaust gas is thus put into a rich air-fuel ratio state wherein oxygen is not present in the exhaust gas, the concentration of the reducing agents is high, and the exhaust gas temperature is raised, the nitrate formed by the occlusion of NOx releases the nitrogen dioxide and returns to the original barium or other catalyst substance. Since oxygen is not present in the exhaust gas, the released nitrogen dioxide is reduced to nitrogen (N2) and purified by the oxidation function of the supported noble metal, using the carbon monoxide, hydrocarbons (HC), and hydrogen (H2) present in the exhaust gas as the reducing agents.
For this reason, when the NOx occlusion ability nears saturation in an NOx purification system provided with the NOx occlusion reduction catalyst, the exhaust gas having a reducing composition is supplied to the catalyst in order to release the occluded NOx and regenerate the catalyst. The exhaust gas having a reducing composition is generated through making an amount of the fuel greater than that in a theoretical air-fuel ratio, thereby putting the air-fuel ratio of the exhaust gas in the rich state, and through decreasing the oxygen concentration of the inflowing exhaust gas. By conducting rich control for recovery of the NOx occlusion ability, the occluded NOx is released and regeneration operation is conducted, by which the released NOx is reduced by the catalyst noble metal.
Additionally, in order to make the NOx occlusion reduction catalyst function effectively, it is necessary to supply in the rich state the amount of the reducing agent necessary and sufficient for reducing the NOx that is occluded in the lean state. In the diesel engine, when the rich state is realized using fuel-related means only, fuel consumption worsens. For this reason, air intake is throttled using a throttle valve while the intake amount is also reduced by opening an EGR valve and supplying a large amount of an EGR gas. In addition, the fuel is added in order to deepen the rich state, and in-cylinder combustion is switched to rich combustion.
Meanwhile, in the internal combustion engines, the burning of the fuel or engine oil causes the sulfur contained in the fuel or engine oil to be produced in a combustion gas as sulfur dioxide (SO2). When the sulfur dioxide present in the exhaust gas reaches the surface of the NOx occlusion reduction catalyst, the sulfur dioxide becomes sulfur trioxide (SO3) and is occluded instead of the nitrogen dioxide, and additionally, adsorbed at the catalyst surface. Under low-temperature exhaust gas conditions wherein the exhaust gas temperature is approximately 600° C. or less and in the normal operating range of the internal combustion engine, the occluded sulfur dioxide is hardly released. For this reason, the occluded sulfur dioxide becomes the cause of sulfur poisoning, wherein the occlusion ability of the NOx occlusion reduction catalyst is worsened.
Consequently, for the NOx occlusion reduction catalyst, it is necessary to periodically control the engine operational parameters, supply the high-temperature and rich-state exhaust gas to the catalyst, and conduct a sulfur purge to release the occluded and adsorbed sulfur from the catalyst and regenerate the catalyst from sulfur poisoning (hereinafter, this process will be referred to as sulfur poisoning regeneration). However, since the high-temperature and rich spike gas is necessary for conducting this sulfur poisoning regeneration, there is a problem that this leads to a remarkable worsening of fuel consumption. For this reason, it is desirable to conduct the sulfur poisoning regeneration only as frequently as is necessary.
As one NOx purification device of the related art, an exhaust gas purification system for an internal combustion engine has been proposed such as that disclosed in Japanese Patent Application Kokai Publication No. 2000-51662, for example. In this device, the amount of sulfur oxides (SOx) occluded by an occlusion type NOx catalyst (NOx occlusion reduction catalyst) is accurately estimated. In order to efficiently eliminate the occluded SOx, the degree of occlusion of sulfur components is computed according to at least one from among the air-fuel ratio, fuel properties, and catalyst temperature. The deposited amount of sulfur components is then estimated based on a value correlated to a fuel injection amount and the degree of occlusion of sulfur components.
However, in this exhaust gas purification system for an internal combustion engine, estimation is conducted indirectly on the basis of the state of the fuel, and thus estimation is not conducted on the basis of the state of the catalyst. Consequently, the degree of catalyst deterioration due to sulfur poisoning cannot be accurately estimated. Therefore, the degree of catalyst deterioration is calculated to be much greater than the actual value, for safety reasons. As a result, the frequency of sulfur poisoning regeneration increases, and this causes excessive worsening of fuel consumption.
On the other hand, an exhaust gas purification device for an internal combustion engine has been proposed such as that disclosed in Japanese Patent Application Kokai Publication No. 2004-60518, for example. In this device, an NOx concentration sensor is disposed in the exhaust passage downstream of the NOx catalyst. Upon resuming NOx occlusion after shifting from an NOx purge mode, based on the time rate of change in an NOx concentration detected by the NOx concentration sensor, when the time rate of change in the NOx concentration is larger than a reference value, the deterioration of the NOx catalyst is taken to be due to sulfur poisoning. In addition, when the time rate of change in the NOx concentration is smaller than the reference value, the deterioration of the NOx catalyst is taken to be due to thermal deterioration. Subsequently, when the deterioration is due to sulfur poisoning, the sulfur purge is conducted. However, there is a problem that the NOx concentration sensor is still expensive, and for this reason, it is difficult to use the NOx concentration sensor in mass-produced products.    Patent Literature 1: Japanese Patent Application Kokai Publication No. 2000-51662    Patent Literature 2: Japanese Patent Application Kokai Publication No. 2004-60518